Investigators said Ireland has become an attractive option because illegal immigrants are able to take advantage of the "soft" border with Northern Ireland and then make their way to the British mainland.

But he also warned that people smugglers were increasingly using Belgium as an easy route to get migrants into the UK.

Mr Dowdall said improved security in France meant they were targeting lorries across the border in Belgian towns and cities.

He explained: "Our area of concern is the fact that organised crime groups are often basing themselves or leading migrants into vehicles in Belgium rather than France and then moving them across the Belgium-France border and then they make their way to the Channel ports.

"That's an area of concern for us. Part of the reason why they do that is part of the sophistication of how they look to operate.

"They think moving from country to country makes it more difficult for law enforcement to coordinate their activity."

Mr Dowdall also said there was evidence of Eastern European lorry drivers being increasingly complicit in smuggling migrants through the traditional Channel tunnel route from Calais to Dover for a fee.

Smugglers target particular spots. Those are nexus points that organised criminals usually use.

"Sometimes they threaten the drivers. Quite often they are finding drivers who are prepared, for a substantial sum of money, to bring migrants to the UK.

"There are Ukrainians and others who will do this.

"I would suggest they are complicit drivers - they are the largest service providers required by organised criminal groups.

"Some drivers will deliberately not have security on their vehicle - cords, padlocks.

"They are prepared to take those risks and organised crime groups will target them because they also depend on them.

Desperate migrants are exploited by people traffickersCredit:
Reuters

Mr Dowdall said that the use of vessels to transport migrants over the Channel was the 'exception to the rule' as the majority of migrants are being smuggled over in vehicles.

The people smuggling industry is believed to be worth around £6 billion annually, with migrants paying gangs at least £6,000 for passage to Europe.

The National Crime Agency's Operation INVIGOR has seen an overall reduction of illegal immigrants since it was set up two and a half years ago, but said that this year has seen an increase year-on-year.